In short this type of filter reduces the brightness of the highlights to make it easier to photographing a high contrast scene. A graduated neutral density filter is, in terms of contrast, divided in two halves. When light strikes this filter one half reduces the brightness by absorbing some of the light and the other half lets the light pass through with little or no absorption. This filter works well when the scene is divided in two halves with a reasonably straight line, either horizontally or vertically such a sunset scene at a beach. If your subject spans both halves of the scene, a  graduated neutral density filter may not be a good choice. You should use a fill flash.

Figure 4 - Neutral Density Filters are either  square or circular in shape. The circular shaped filters can be directly screwed onto the lens. But, the square shaped filters usually come with a filter holder that attaches to the lens and holds the filter in place. The top portion of the filters shown in this figure, blocks some of the light to reduce the contrast of the overall scene.

Taking a step deeper into the darkness of the world of night photography will bring a new set of challenges. Once again, let's remind ourselves of the fact that the exact amount of light for correct exposure, for a given film speed depends on the aperture opening and the time period during which the shutter remains open. At night, the available light is much less than what we get during bright daylight or when the sun is rising or setting. Hence, usually the shutter needs to remain open in the order of seconds. Alternatively, you can use faster films (ISO number 800, 1600, 3200, and so on). Faster films do not give you this flexibility without a price. Usually, pictures produced from this type of films are grainier and cannot be enlarged beyond a certain size.

It is also important to note that at night we get the light from a variety of sources such as moonlight, tungsten flood lights, neon lights, florescent lights, and so on. Each of these types of light have a different color temperature and exposes the film differently. For example, daylight has a color temperature of between 5000^oK and 6000^oK and light produced by tungsten lights has a color temperature of about 3200^oK. Since most of the films are designed for day light (we will discuss various types of films for different lighting scenarios in a separate article), if exposed under tungsten light source will produce a reddish orange tone (see figure 5 below), and produce a sickly green tone when exposed under florescent lights.

Figure 5 - A daylight balanced film exposed to a tungsten light source produces a reddish orange tone throughout the picture. This problem can be eliminated by either using tungsten balanced film or a corrective blue filter.

In some shooting scenarios these tones can be considered special effects. If you are shooting a scene that requires daylight and all you have is one of the light sources mentioned above, you can use special corrective filters. These filters are available for various light sources. We will cover these filters and what they can do in a separate discussion.

Since long exposures are often used in night photography, it is important to keep the camera steady. Therefore, a good sturdy tripod is a must. After you determine your composition you can take a reading with your camera's internal light meter ( an average reading of the scene would do just fine). Depending on the type of camera you have you may need to do some calculations to come up with the final aperture and shutter speed settings. If you have one of the new semi-high end cameras or better, you're in luck. Most of these cameras have shutter speeds as slow as 30 seconds and most of the nightlight photography scenarios do not require shutter speed slower than 30 seconds. If you come across a situation that does require longer exposure, you can use the bulb setting. Due to film reciprocity problems which will complicate the calculation, we do not recommend very long exposures (reciprocity will be covered in a separate discussion on films). If you have an old, low end, or a manual camera that probably doesn't have a shutter speed slower than 2 seconds, not to worry. Here is an example to help you deal with this problem - you may alter it to suit your situation. Let's say you are using a film with ISO number 100. When you set the aperture to f/stop 5.6 and the lighting situation requires a shutter speed of slower than 2 seconds your camera will not be able to determine the exact shutter speed required. You can keep the same aperture setting of 5.6  and increase the camera setting to ISO number 200, 400, 800, and so on until the shutter speed required is 2 seconds or less. Let's say for an ISO 800 setting with aperture set to 5.6 you require a shutter speed of 2 seconds. Therefore, for a film with ISO number 100 and aperture setting of 5.6 you need the shutter to stay open for 16 seconds ( 2 seconds x (800/100)  = 16 seconds). Once you have determined the duration of the exposure, you can use a bulb setting to keep the shutter open for the duration of the exposure (using a handheld timer or by counting starting at 1001, 1002, and so on...). Do not worry if you cannot exactly time or determine the shutter speed exactly. Sometimes, you will be pleasantly surprised by the results. Just make sure there is not too much contrast in the scene (see figure 6 on the right side). If the composition permits, you can use a Graduated Neutral Density Filter. You can also try eliminating the bright spots from the composition.

Figure 6 - This night scene of the San Diego Bay was shot with the aperture wide open and the shutter speed longer than 12 seconds.

When photographing directly into any light source, you risk getting lens flares or a star effect in your pictures. Leaving the aperture wide open (smallest f/stop allowed by the lens) will prevent the star effect and reduce the chance of  lens flares. See figure 7 below).

Figure 7 - When the aperture is stopped down in long exposure pictures, it will cause a star effect around each light source. To avoid this problem (if you think this is a problem) leave the aperture wide open.

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